The present invention provides a method and related apparatus for separating or classifying ultra-fine or nanometer-sized powder particles. The method and apparatus are effective in separating and classifying various nano-sized powders, which can be used in industrial or consumer products such as abrasives, chemical catalysts, agricultural chemicals, animal feeds, carbon and graphite, cement, ceramics, clay, coal and coke, construction materials, cosmetics, detergents, fertilizers, fillers, frits, enamels and glazes, food products and colorings, herbs and spices, industrial and specialty chemicals, insecticides and pesticides, marine feeds, metallic minerals and ores, metallic powders, oxides and compounds, minerals (non-metallic), paints, pigments and dye stuffs, pharmaceuticals, pulverized fuel ash, rare earth metals and compounds, refractory materials, resins and waxes, slags, surface coatings, and toners.
Particle separators or classifiers for ultra-fine solid powders have a tremendous utility value. This is due to the unusually wide range of applications that ultra-fine powders, including nanometer-sized powders, have enjoyed. Nano-sized powders are essential ingredients in a broad array of both industrial and consumer products, listed above. In most of these applications, particles of well-defined sizes and/or a narrow size distribution are highly desirable for improved product performance.
Additionally, nano-sized metal powders are being considered for use as primers, propellants, and high explosive energetic materials. The particle size uniformity and homogeneity of particle mixing are two critical factors that hold the promise of further improving the performance of these metal powders. However, no method currently exists to guarantee the particle size uniformity in the desired range of nanometer sizes. Conventional mechanical methods of separation (e.g. metal screen sieves) are not feasible for separating particles at the nanometer scale. Current electrostatic charge and air-current methods are not capable of providing classification of nanometer-sized particles of an ultra-narrow size distribution as may be required of highly efficient and reliable energetic materials. An urgent need exists for an innovative method and equipment that are capable of precisely classifying nanometer-sized particles into groups of very narrow size ranges at a good production rate.
The following patents are believed to represent the state of the art of powder classifiers:
1. H. Morimoto, et al., xe2x80x9cAir Current Classifying Separator,xe2x80x9d U.S. Pat. No. 6,269,955, Aug. 7, 2001.
2. S. Akiyama, xe2x80x9cPowder Classifier,xe2x80x9d U.S. Pat. No. 5,931,305, Aug. 3, 1999.
3. W. A. Howell, xe2x80x9cDust-free Powder Substance Delivery and Filter System,xe2x80x9d U.S. Pat. No. 5,518,343, May 21, 1996.
4. H. Kanda, xe2x80x9cGas Current Classifying Separator,xe2x80x9d U.S. Pat. No. 5,165,549, Nov. 24, 1992.
5. M. Kato, et al., xe2x80x9cAir Current Classifier, Process for Preparing Toner, and Apparatus for Preparing Toner,xe2x80x9d U.S. Pat. No. 5,016,823, May 21, 1991.
6. Y. Yamada, et al., xe2x80x9cPowder Classifier,xe2x80x9d U.S. Pat. No. 4,604,192, Aug. 5, 1986 and U.S. Pat. No. 4,560,471, Dec. 24, 1985.
7. N. Nakayama, xe2x80x9cAir Classifier,xe2x80x9d U.S. Pat. No. 4,221,655, Sep. 9, 1980.
8. Y. Sogo, xe2x80x9cCyclone Separator,xe2x80x9d U.S. Pat. No. 4,149,861, Apr. 17, 1979.
9. J. Drew, et al., xe2x80x9cCentrifugal Separator Apparatus,xe2x80x9d U.S. Pat. No. 3,753,336, Aug. 21, 1973.
10. B. G. E. Mansson, xe2x80x9cApparatus for Separating Solids in a Whirling Gaseous Stream,xe2x80x9d U.S. Pat. No. 3,643,800, Feb. 22, 1972.
11. B. N. Hoffstrom, xe2x80x9cRotary Flow Classifier,xe2x80x9d U.S. Pat. No. 3,334,741, Aug. 8, 1967.
12. J. D. Miller, E. E. Koslow, K. W. Williamson, U.S. Pat. No. 4,676,807, Jun. 30, 1987 and U.S. Pat. No. 4,759,782, Jul. 26, 1988.
13. J. G. Billingsley, et al. xe2x80x9cCyclone Separator,xe2x80x9d U.S. Pat. No. 5,236,479, Aug. 17, 1993.
14. A. Matsui, xe2x80x9cDust Collector Adapted for Use in a Hopper Dryer,xe2x80x9d U.S. Pat. No. 4,848,990, Jul. 18, 1989.
15. C. Davis, xe2x80x9cLow Pressure HEPA Filtration System for Particulate Matter,xe2x80x9d U.S. Pat. No. 4,490,162, Dec. 25, 1984.
16. H. J. Obermeier, xe2x80x9cDual Cyclone Dust Separator for Exhaust Gases,xe2x80x9d U.S. Pat. No. 4,406,677, Sep. 27, 1983.
17. H. J. Lader, xe2x80x9cSystem for Controlling and Utilizing Finer Powder Particles in a Powder Coating Operation,xe2x80x9d U.S. Pat. No. 5,454,872, Oct. 3, 1995.
18. S. Masuda, xe2x80x9cElectric Dust Collector Apparatus,xe2x80x9d U.S. Pat. No. 3,985,524, Oct. 12, 1976.
19. S. Nishikiori, et al. xe2x80x9cCyclone Type Dust Collector,xe2x80x9d U.S. Pat. No. 6,042,628, Mar. 28, 2000.
20. S. Minakawa, xe2x80x9cCyclone Dust Collector,xe2x80x9d U.S. Pat. No. 5,948,127, Sep. 7, 1999.
21. B. G. Jung, xe2x80x9cDust Collector Using Purse-Type Filter Cloth,xe2x80x9d U.S. Pat. 5,683,477, Nov. 4, 1997.
As indicated in the-above-cited patents, various techniques for separating and classifying powders have been proposed. One of such conventional techniques, known as powder classifier, provides a rotor for classifying powders by using the rotation of the rotor and airflow. The rotor spins at a high speed inside a casing with the rotor being equipped with a plurality of powder classifying vanes swirling around, while ventilating the rotor from the periphery to the center. The airflow and the centrifugal force caused by the rotation act on the powder flow to classify the powder particles in accordance with the boundary defined by a desired particle size.
More specifically, an air introduction path is formed to be directed toward the inside of the rotor from the position where the powder classifying vanes are provided, and a powder introduction port or powder intake is formed above the classification rotor along the circumference thereof from which powder particles fall onto the powder classifying vanes. A powder supply port is provided on the upper center of a casing for supplying the powder as a raw material. The powder supplied is fed from the powder intake to the powder classifying vanes within the rotor, i.e., fed into a classification chamber while being scattered on the upper surface of the rotor. In the classification chamber, the centrifugal force of the powder classifying vanes and the air flowing into the center of the rotor act on the powder. In other words, fine powder particles with a small diameter that is very susceptible to air viscous resistance are carried by the airflow to the central portion and taken out from a fine powder outlet, while coarse powder particles having a large diameter that is very susceptible to the centrifugal force are scattered to the outer edge of the classification rotor by the centrifugal force and collected to a coarse powder outlet provided on the outer peripheral of the rotor. The powder is thus classified in accordance with the boundary defined by a desired particle size.
Such a conventional powder classifier is also provided with a balance rotor, unitarily with the classification rotor, so that the air passing through the classification rotor is introduced through the balance rotor from the center of the classification rotor into the fine powder outlet provided in the outer edge of the classification rotor. The balance rotor is provided with a view to regulating the flow of air passing through the classification rotor or a vent cavity or ventilating the vent cavity smoothly so that the powder can be classified in accordance with the desired value.
Since in the conventional classifier the balance rotor is coupled to the lower portion of the classification rotor, the flow can be balanced in the vertical direction. Such a balance rotor, however, makes the entire mechanism of the powder classifier complicated and the rotor large scale to increase the weight. The heavy rotor causes an increase in output of a drive mechanism for driving the rotor to rotate. Further, since in the powder classifier the vent path from the classification rotor to the balance rotor is bent substantially at 180 degree and the sectional area of the path is increased from the center to the circumference, the ventilating speed is reduced and hence the classified powder particles could be accumulated or adhere to the inner surface of the vent path. The powder particles adhered may cause lowered permeability or clogging of the vent path. Because the entire mechanism is complicated, it is difficult to disassemble the classification rotor and it takes much time to clean the inside of the classification rotor for keeping its sanitary conditions or remove clogging powder particles from the vent path.
Akiyama, et al [Ref.2] provided a powder classifier using a classification rotor capable of classifying powder with high efficiency and high accuracy. The classification rotor is attached to a rotating shaft as a body and rotatably supported in a casing. Within the classification rotor, a cavity is formed from the outer edge to the center and classifying vanes are provided around the circumference. The cavity is bent downwardly near the center with the lower end connected through a fine powder passage to a fine powder outlet. The outer edge of the classification rotor is connected to a coarse powder outlet. After feeding powder from a powder supply port, the powder is rotated by the classifying vanes such that coarse powder particles are taken out from the rough outlet by centrifugal force and fine powder particles are taken out by airflow from the fine powder outlet.
Morimoto et al [Ref.1] developed a powder classifier to reduce the classification point for classifying powder. The classifier includes a classifying cover having a conical bottom surface, a classifying plate provided under the classifying cover and having a conical top surface opposite the conical bottom surface of the classifying cover, and a plurality of louvers provided annularly around a classifying chamber defined between the conical bottom surface and the conical top surface to define passages for secondary air. The conical bottom surface is inclined at a larger angle than the conical top surface.
Kanda, et al [Ref.4] provided a separator for classifying powder with air current. The separator includes a classifying chamber and an introduction section for introducing powder into the classifying chamber, a powder feeding inlet for feeding powder formed at the upper portion of the classifying chamber, a cone-shaped classifying plate with a high central portion formed at the lower portion of the classifying chamber, a coarse powder discharging outlet for discharging coarse powder provided at the lower brim outer periphery of the classifying plate, a fine powder discharging outlet for discharging fine powder provided at the central portion of the classifying plate, a gas in-flower for dispersing powder by whirling gas provided at the upper outer periphery of the classifying chamber, and a gas inflow inlet for creating a whirling current of gas for classifying powder provided at the lower portion of the classifying chamber. When the powder starting material flowing into a classification chamber is fluidized in a whirl in said classification chamber, centrifugal force and air resistance force in the inward direction act on the respective particles of the powdery starting material, and the classification point is determined by the balance between the centrifugal force and the air resistance force.
At the outer periphery of the classification chamber, larger particles are whirled, while smaller particles whirl inside thereof. By providing powder-discharging outlets respectively at the center and the outer periphery of the lower portion of the classifying chamber, the fine powder group and the coarse powder group can be collected separately (classification). In such a classifying separator, it is important that the starting powder should be sufficiently dispersed within the classifying chamber to become primary particles in enhancing the classification precision. As this kind of classifying separator, an litani system classifying separator or Kuracyclon has been proposed. However, in this type of classifying separator, it is very difficult to control the classification point, to and involves such problems such as poor dispersion and poor classification precision when there is high dust concentration. In order to solve such problems, various proposals have been made [e.g., Ref.6]. As a classifying separator practically applied, there may be mentioned a commercially available classifying separator sold under the name of DS separator. In this kind of classifying separator, although it has become possible to control the classification point, since powder is fed through a cyclone section into the classifying chamber, the powder is concentrated before entering the classifying chamber, whereby dispersion of the powder tended to become insufficient.
The result of a through literature search indicates that existing powder classifiers or separators are not effective in classifying powder particles smaller than 10 microns. Most of the commercially available separators are not designed for or capable of separating nanometer-sized powder particles at all. An urgent need exists for the development of both general-purpose and highly specialized nano powder separators that are of good accuracy.
Therefore, an object of the present invention is to provide a method and related apparatus that are capable of separating nanometer-sized powder particles.
Another object of this invention is to provide a method and apparatus for classifying a powder into separate groups of nanometer-sized particles with at least one group consisting of only particles within a very narrow size range.
Still another object of this invention is to provide a multi-stage powder separator apparatus that is capable of classifying a nano powder into several groups of nanometer-sized particles with each group consisting of particles within a narrow size range.
As one of the preferred embodiments of the present invention, a nano powder-separating or powder-classifying method includes:
(a) feeding the powder particles into a pressurized gas stream which carries the particles into a first stage filter device of a multiple-stage separator system;
(b) operating the first stage filter device to remove and collect coarse particles and a filter device in at least another stage to remove and collect finer particles of the powder; the filter device having a dynamic filter which is composed of (b1) a mesh of a multiplicity of openings with the opening size at least two times larger than the average size of the particles, (b2) vibration devices or shakers to shake off the particles that may otherwise clog up the mesh openings, (b3) size sensors to measure the sizes of the particles collected by the filter devices, and (b4) a controller to regulate the operations of the shakers and sensors: in order to form desired dynamic mesh holes for the purpose of filtering out the coarse particles in the first stage or the finer particles in another stage; and
(c) operating a dust collector to exhaust the residual gas, allowing the finest particles of the powder to be separated and collected.
Preferably, the particle size signals acquired by the sensor are fed back to the controller for the purpose of adjusting the operation, on demand, of the vibration devices or shakers to achieve the desired dynamic mesh holes. Further preferably, the shaking motion of the vibration devices is regulated by the controller to vary the amplitude, frequency, direction, and/or waveform of the shaking motion to achieve the desired dynamic mesh holes. The feeding rate of the powder particles is preferably adjustable under the command of the controller. The multiple stage filter devices are operated in a closed-loop control fashion that powder particles whose diameters, d, fall within a narrow range, dminxe2x89xa6dxe2x89xa6dmax, can be readily collected. Preferably, at least a collector container is capable of collecting particles where (dmaxxe2x88x92dmin)xe2x89xa650 nanometers. Further preferably, the particles are very narrow in size distribution: (dmaxxe2x88x92dmin)xe2x89xa620 nanometers.
In one of the preferred embodiments, a multiple-stage powder separator apparatus for separating nanometer-sized particles of a powder is composed of the following major components:
(a) a powder feeder;
(b) at least a first stage filter device in flow communication with the powder feeder to receive powder particles therefrom; the filter device including
(b1) a casing,
(b2) at least a flexible filtering mesh inside the casing with mesh openings at least two times larger than the average size of the particles to be separated; the filtering mesh and the casing together forming a first outer cell therebetween and a first inner cell inside the filtering mesh, the first inner cell being in flow communication with the powder feeder;
(b3) a rotor equipped with a plurality of powder classifying vanes being inside the inner cell and swirling around an axis of this rotor, which is driven by a first motor; the swirling vanes driving fine particles smaller than a predetermined size to permeate through the mesh openings to enter the first outer cell, leaving behind coarse particles inside the first inner cell;
(b4) vibration device in shaking relation to the flexible filtering mesh to form dynamic mesh holes;
(b5) a controller in control relation to the vibration devices;
(b6) a first powder collector in flow communication with the first inner cell to receive the coarse powder particles therefrom;
(b7) particle size sensors, in electronic communication with the controller, to measure the sizes of the particles and feed the acquired size signals to the controllers through an amplifier-driver unit; and
(c) a dust collector in flow communication with the at least first stage filter device to receive the fine particles therefrom. The dust collector is composed of a dust filter to filter out finer particles and a collector container to collect the finer particles, permitting the residual gas to exhaust through the dust filter.
Preferably, the above-described apparatus further includes at least a second stage filter device in flow communication with the first stage filter device on one end and with the dust collector on another end of the at least a second stage filter device (can have 3, 4 or any number of stages as desired). The second stage filter device preferably has a similar construction as the first stage one, also including a casing, a flexible filtering mesh, a rotor, a shaker, a size sensor, and a powder collector container. This second particle size sensor is used to determine the sizes of relatively larger-sized particles (that are not able to permeate through the flexible filter mesh in the second stage) and feed the acquired size signals to the controller.
The powder feeder is preferably composed of a hopper receive the powder, a feeding gear with one end being in flow communication with the hopper to receive powder particles therefrom and another end to output particles at a desired rate, a pressurized air inlet communication with the output end of the feeding gear to receive powder particles therefrom for forming a powder-gas mixture stream that enters the inner cell of the first stage filtering device. This feeding gear is preferably under the command of the controller so that the powder feeding rate can be adjusted in real time during the powder separating process. Preferably, the casing and the inner cell in each filter device is approximately conical in shape, tapering down from a larger upper-portion diameter to a smaller lower-portion diameter. The controller preferably includes an amplifier and driver unit for driving the vibration devices with adjustable vibration amplitude, frequency, direction, and/or waveform.